The majority of all polymers produced by polymerization of ethylenically unsaturated monomers are prepared with the aid of ingredients, called free-radical catalysts or initiators, which provide for the initiation and propagation of the growing chains through a free-radical mechanism. Such catalysts are compounds, generally organic compounds, that under the conditions of the reaction, slowly decompose and produce fragments which carry with them an unbonded or unpaired (lone or free) electron. This unbonded electron, sometimes called an or orbital electron, reacts with the electrons of a colliding monomer to form a covalent sigma bond (the initiation process), with the reproduction of a new unpaired electron which is capable of repeating this process and propagating a chain. In the light of this mechanism, the catalysts are free-radical formers or free-radical initiators.
Free-radicals have been generated in a number of ways, including thermal or photochemical decomposition of such compounds as organic peroxides or hydroperoxides, or azo or diazo compounds. Two well known reactions commonly used to produce free-radicals for polymerization include the decomposition of benzoyl peroxide and of azobisisobutyronitrile.
The stability of free-radicals varies over a wide range. Primary free-radicals are more reactive than secondary free-radicals, which in turn are more reactive than tertiary free radicals. Phenyl free-radicals are more reactive than benzyl free-radicals. Allyl free-radicals are quite unreactive.
Reaction rate control of free-radical initiated polymerization is sometimes troublesome because, once initiated, such reactions tend to progress rapidly, creating undesirable exotherms and limiting the scope of the polymeric product types. Such polymerization reactions are carried out most commonly in an organic phase because the monomeric materials, being organic, are soluble in organic reaction media. Some reactions are carried out in an aqueous phase, mainly as emulsions.